The present invention relates to gas turbine engine combustion systems, and more particularly to a staged combustion system in which the production of undesirable combustion product components is minimized over the engine operating regime.
Modem day emphasis on minimizing the production and discharge of gases that contribute to smog and to other undesirable environmental conditions, particularly those gases that are emitted from internal combustion engines, have led to different gas turbine engine combustor designs that have been developed in an effort to reduce the production and discharge of such undesirable combustion product components. Other factors that influence combustor design are the desires of users of gas turbine engines for efficient, low cost operation, which translates into a need for reduced fuel consumption while at the same time maintaining or even increasing engine output. As a consequence, important design criteria for aircraft gas turbine engine combustion systems include provision for high combustion temperatures, in order to provide high thermal efficiency under a variety of engine operating conditions, as well as the minimization of undesirable combustion conditions that contribute to the emission of particulates, to the emission of undesirable gases, and to the emission of combustion products that are ,precursors to the formation of photochemical smog.
Various governmental regulatory bodies have established emission limits for acceptable levels of unburned hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), which have been identified as the primary contributors to the generation of undesirable atmospheric conditions. And different combustor designs have been developed to meet those criteria. For example, one way in which the problem of minimizing the emission of undesirable gas turbine engine combustion products has been attacked is the provision of staged combustion. In that arrangement, a combustor is provided in which a first stage burner is utilized for low speed and low power conditions, to more closely control the character of the combustion products, and a combination of first stage and second stage burners is provided for higher power outlet conditions while attempting to maintain the combustion products within the emissions limits. However, balancing the operation of the first and second stage burners to allow efficient thermal operation of the engine, on the one hand, while on the other hand simultaneously minimizing the production of undesirable combustion products is difficult to achieve. In that regard, operating at low combustion temperatures to lower the emissions of NOx, also can result in incomplete or partially incomplete combustion, which can lead to the production of excessive amounts of HC and CO, in addition to producing lower power output and lower thermal efficiency. High combustion temperature, on the other hand, although improving thermal efficiency and lowering the amount of HC and CO, often result in a higher output of NOx.
Another way that has been proposed to minimize the production of those undesirable combustion product components is to provide for more effective intermixing of the injected fuel and the combustion air. In that regard, numerous mixer designs have been proposed over the years to improve the mixing of the fuel and air so that burning will occur uniformly over the entire mixture, to reduce the level of HC and CO that result from incomplete combustion. On the other hand, even with improved mixing, under high power conditions, when the flame temperatures are high, higher levels of undesirable NOx are formed.
Thus, there is a need to provide a gas turbine engine combustor in which the production of undesirable combustion product components is minimized over a wide range of engine operating conditions.
It is therefore desirable to provide a gas turbine engine combustion system in which staged combustion can occur, to respond to particular power output demands, and also one in which the emission of undesirable combustion product components is minimized over a broad range of engine operating conditions.
Briefly stated, in accordance with one aspect of the present invention, a fuel nozzle assembly is provided for use in a gas turbine engine. The fuel nozzle assembly includes a primary fuel injector having a central axis, and the primary fuel injector is disposed for injecting a primary fuel spray into a primary air stream. A secondary fuel injector is positioned radially outwardly of the primary fuel injector for injecting a secondary fuel spray into a secondary air stream that is spaced radially outwardly of and that surrounds the primary air stream. At least one air jet is positioned between the primary fuel injector and the secondary fuel injector and is inclined relative to the primary fuel injector central axis to direct a portion of an incoming air stream between the primary air stream and the secondary air stream in an angular downstream direction relative to the primary air stream.